A smoking device made out of either a 2 or 3 liter bottle and a bucket of water using "Grav"ity to fill the bottle with smoke and to race the smoke into your lungs. Smoke from a grav bong ranges from skim milk (bitch hits), to whole milk (yellow chalky one-hit quitters). Great for those who like making their own pieces and those who want to experience an extreme high (greater than the first time you smoked a bong).